Since 2000, high-speed communications have rapidly proliferated due to ADSL (Asynchronous Digital Subscriber Line) services for which metal cables are used, changing the broadband services to high-speed Internet access. Further, FTTH (Fiber To The Home) high-speed Internet access has begun to be offered commercially through connection of a node to each home with optical fibers for video transmission services like CATV lines. Today's FTTH access networks support high-speed and broadband services with bidirectional transmission over a single optical fiber to each optical network unit (ONU) (optical transceiver module) which is an optical subscriber line terminal installed on a telephone pole or within an apartment building such as a condominium. To further expand the FTTH, it is necessary to lower the connection fee-s by reducing the equipment/installation costs. In particular, it is demanded to reduce the size and cost of ONUs which occupy a large part of the cost.
The FTTH optical access network is characterized by its transmission system called WDM (Wavelength Division Multiplexing) transmission. Specifically, optical signals from a plurality of optical sources (for example, laser diodes) of different wavelengths are multiplexed into a single composite optical signal by a wavelength division multiplexing module on the transmitter side. The single composite optical signal is entered into a single optical fiber. On the receiver side, a wavelength division demultiplexing module demultiplexes the single composite optical signal according to the wavelengths before conversion to electrical signals by photo detectors (for example, photodiodes). In addition, the WDM (Wavelength Division Multiplexing) transmission system can easily realize bidirectional transmission, allowing a single optical fiber to transmit signals in two directions simultaneously. For example, in one direction, an optical signal of wavelength λ1 enters from the node into the optical fiber and is received by a photo detector on the access side. In the other direction, an optical signal of wavelength λ2 enters from the access side into the optical fiber and is received by a photo detector at the node. In this case, each of the node and the access side has a wavelength division multiplexing/demultiplexing module (bidirectional optical module). In recent years, since three services, namely telephony, video and data services have become popular, the ONU is required to support multiple wavelength bidirectional transmission.
Thus, the ONU (optical subscriber line terminal) is provided with an optical transceiver module capable of multiplexing and demultiplexing multiple wavelengths. For example, an optical transceiver module disclosed in Japanese Patent Laid-Open No. 2000-180671 (Patent Document 1) comprises a wavelength division multiplexing filter to multiplex/demultiplex two wavelengths, a condenser lens to efficiently couple an optical signal from an optical fiber to a passive optical component and a condenser lens to efficiently couple an optical signal from a laser diode to the optical fiber. This type of optical transceiver module are also disclosed in Japanese Patent Laid-Open No. 2005-45635 (Patent Document 2) and Japanese Patent Laid-Open No. 2005-84188 (Patent Document 3). Each of them is designed so as to reduce the number of components.
[Patent Document 1] Japanese Patent Laid-Open No. 2000-180671
[Patent Document 2] Japanese Patent Laid-Open No. 2005-43635
[Patent Document 1] Japanese Patent Laid-Open No. 2005-84188